So this technique is some sort of photoshop matte painting.
Basically you start with a 3d render (or a 2D painting), then you use 2D painting
(or texture) with incrustation mode over it with somes layer adjustements.
Sometimes I also add some low opacity normal layer of the texture, and a low
opacity lightning layer.

I used 4 images from mr_carl as layers, they all got the same size that helped me a lot:
*A*: The 3D image with a complete modeling on the dragon and character, and with colors.
*B*: The 2D painting with only the dragon and the character on a white background I erased.
*C*: The cold colors 3D second background (the ones with skyscrapper boxes and reflexion).
*D*: The final 2D painting with the filtering.

The first step: The foreground:

-1: The *A* layer on 100% normal, it's my 3D base layer, the main one.
-2: It's just the dragon's horn part from the *B* layer (30% normal), it's just because
the 3d layer had no horn, this layer is just detail.
-3: Again the horn part with a piece of the dragon's stomach, and a piece of the dragon's mouth
from *B*. It's on 74% product. It gives some consistance to the horn, and shadows on the stomach
and mouth. This layer is just detail.
-4: The *B* layer on 60% incrustation, this layer is important, you mix the 2D with the 3D here.
I added a mask to lower a little bit the opacity of some 2D zones.
-5: The *B* layer on 20% crude light just on the dragon part, that's how I get back the volumes
from the 2D painting *B* layer.
-6: The *B* layer on 50% tint, that's how I get back some colors from the 2D painting *B* layer,
but it's not really effective here, this layer is just detail.

*E*: The collapsed image result from the first step, but just the dragon and the character, I erased the
background.

The second step: The background:

-2: The *C* background 3D layer on 100% normal, it's my 3D background base layer.
-4: The *D* painting on 30% product, it gives some consistance for the next layer.
-5: The *D* painting on 60% incrustation, an important layer, I start to mix the 2D painting with
the 3D here.
-6: The *C* 3D background on crude light 60%, it's because I want to keep the 3D lightning over the
painting. The added mask go with the patch, I'll explain about it later.
-7: The *D* painting layer on normal 85% with a mask. this mask allow me to control the opacity
of the 2D painting parts I want to merge on the 3D. It's an important layer. (You can also switch this
layer with 6, you get a better 3D background, or duplicate it to sandwitch the 6 crude light layer with
two mid opacity normal layer)
-8: It's a black shadow product on 45% of the character and the dragon.
I used *E* (the first step image result), reducing luminosity twice to get it black, and a vertical
scale transformation. Finally I used a mask to project the shadow on the ground only.
-9: The *D* final painting on 100% normal, but just the character and the dragon, not the background part,
the added mask is because I erased a little bit the rising and flying stone from this layer. This is my base
layer for the foreground integration.
-10: The *E* result layer on 65% normal, I used a mask on the stone again. This is the link between first
and second step.
-11: A free 100% normal layer with some black and white low opacity touch for lightning details on the dragon,
not really important.
-12: The *E* result layer on crude light 25%, but just the dragon's head using a gradient mask, this layer
gives volume and add details on the dragon's head, because this last one is more on the foreground than the
character, I think so...
-3: It's a low luminosity piece of *D* on normal 75%, it's some sort of hiding patch on the
boxes near the dragon's head, it's because those boxes mess with the dragon's head's contour.
-1: The left frame is from a scaled piece of the left border, it's a copy from all the collapsed layers of
the second step. Now the image is larger (maybe too much) but I think I got a better diagonal dynamic with
the dragon. The right frame piece is for balance, this layer is just composition details. (100% normal)

I finished collapsing the whole image, duplicating it for a 30 or 40% superposition layer (don't remember),
collapse, reducing the saturation, some details correction, and playing with the level correction because I
think it was too dark, that's all.

I know maybe it seems complicated but it's mostfully playing with normal, incrustation and crude light layers mode.
If you've got any questions do not hesitate, I've got my internet connection back.
I've lost somes interesting details of the 2D painting during the process, there are others corrections to add
to the image, the mixing technique is not perfect... What do you think about it?

Hi washka and sorry for the late reply but that Dominance war is taking up alot of time.
Anyway, I have read through what you have done a coupple of times and you´re final image does bring a bit more the the hole, it has more depth and the lightnign looks really good, but just as you said some of the detales from my final image have been destroid, but thats easy to fix i think.
I actually dont have so manny questions be cause you explaind really good how you did what you did. I will have to read it through some more times and try it out with my beuty shot in DWIV whan the time comes.
Thanks for the effort.